The digital transmission of images requires a considerable transfer of data. Therefore, various coding techniques are used so as to reduce the quantity of data transmitted, thus increasing the speed of transmission and of reading of these images.
Certain techniques, such as the MPEG4 standard, use a so-called multiresolution technique. According to this technique, the coding information in respect of an image is divided into as a first main signal, or “layer”, and a second improvement signal, or “layer”.
The obtaining of these two layers is explained with the aid of the description of FIG. 1 which is a diagram of a video image coder 10 using the MPEG4 standard.
First, this coder 10 comprises a first branch 11 receiving a signal 12 of an image 12i to be coded.
A first processing of the signal 12 consists in subtracting from this signal 12 the information already transmitted in respect of a previous image 12j. This inter-image processing is applied when the image 12i is coded with the aid of information contained in the previous image 12j.
This operation is performed with the aid of a subtractor 14 which for this purpose receives a signal 34 corresponding to the information transmitted in respect of the previous images. The obtaining of this signal 34 is described subsequently.
Thus, the signal 12 is transformed into a signal 1614, the latter then being directed to a transformer 18.
This transformer 18 transforms the signal 1614, defined in the spatial domain, into a signal 1618 defined in the frequency domain. This operation, which is performed without loss of information, is a discrete cosine transform or DCT.
The signals 1618 are transmitted to a quantizer 20 which reduces the dynamic range of these signals by determining their quantization interval. This quantization involves approximations causing a non-negligible loss of information of the signal 1618. The information thus lost during this quantization operation is referred to as the residual, whereas the information retained after quantization constitutes the main layer.
Stated otherwise, the main information layer comprises the data received by the coder minus this residual.
The residual is transmitted by the improvement layer as described subsequently.
A coder 22, for example a Huffman coder, allows a further reduction in the quantity of information to be transmitted.
On exit from the quantizer 20, the signal 1620 is also transmitted to a subtraction loop, which comprises an inverse quantizer 24 performing the inverse function of the quantizer 20. This inverse quantization operation is performed without any new loss of information.
A signal 1624 is then obtained which is applied to a converter 26 performing the inverse function (IDCT) of that carried out by the transformer 18, that is to say converting this signal 1624 from the frequency domain to the spatial domain, thus delivering a signal 1626 at its output.
This signal 1626 is transmitted to a memory 28 and a motion estimator 30, then to the subtractor 14.
The signal 1624 is also transmitted to a subtractor 36, forming part of a branch 35 for processing the improvement layers. A second input of the subtractor 36 receives the output signal 1618 from the DCT transformer 18.
This subtractor 36 therefore performs the subtraction between signals representing the signals received 1618 and transmitted 1624. Thus, the residual 1636 is obtained at the output of the subtractor 36.
The branch 35 includes a memory 38 which stores the residuals (in the spatial frequency domain) as frames and a device 40 which performs the splitting into improvement planes of the signal 1638 at the output of the memory 38 according to the standardized so-called “Fine granularity scalability” (FGS) process.
Each of the improvement planes comprises residual data which are complementary to one another and to those transmitted by the main layer. These planes are ranked by priority according to the improvement of the resolution that their transmission engenders.
For example, consider the reception of an image I composed of a main layer C1 and of an improvement layer C2 comprising three planes P1, P2 and P3 such that P1 has priority over P2, the latter having priority over P3.
So, on reception of the main layer C1, this image I can be obtained with a specified resolution. If the improvement layer transmits the improvement plane P1, this image will exhibit better resolution. If the improvement layer also transmits the plane P2, the resolution will be even better. The best resolution will be obtained if the plane P3 is also used.
However, the improvement of the resolution is less and less noticeable as the priority of the planes transmitted decreases.
With this process, the higher the resolution of an image the greater the transmission delay or reading delay on account of the increased number of improvement planes transmitted.
This is why, in order to improve the speed of coding, transmission or reading of an image, it is known practice to code with different resolutions—that is to say with different numbers of improvements planes—the various zones of one and the same image.
It is therefore possible to apply different resolutions in respect of the different zones of an image.
The time taken to code, transmit or read an image is therefore reduced by reducing the resolution of zones of the image, referred to as background zones, considered to be less important than other zones, referred to as zones of interest, whose resolution is kept high, that is to say whose residual is transmitted in full.
The term high resolution will be used for the resolution of the zones of interest, that is to say with complete transmission of the residual, and the term low resolution will be used for the resolution of the background zones, that is to say with incomplete transmission of the residual.
For example, in the case of an image representing a bird flying over a totally blue sky background, the resolution of the zone of the image corresponding to the sky can be decreased while retaining a high resolution for the zone of the image relating to the bird without, in theory, impairing the overall quality of the image.
The present invention results from the finding that this processing does not always yield satisfactory results. Specifically, the images thus processed exhibit anomalies of resolution at the boundaries between the zone or zones of interest and the background zone or zones.
The present invention solves this problem. It is based on the observation that the blockwise processing of pixels is unsuitable for the coding of images involving several resolutions.
It is known, in fact, that video data are coded and transmitted as pixel blocks, for example 8*8 blocks for the MPEG2 or MPEG4 standard.